|
It is likely that planets are just a by-product of star formation, stemming from
the physical law of angular momentum conservation. The same
effect can be observed when a spinning skater gradually pulls the arms
closer to the body, thereby turning faster and faster. Stars form through the gravitational collapse of interstellar
matter. During this process, the diameter of the affected cloud of
matter shrinks by a factor of more than 100 million. In the presence
of rotation and/or a magnetic field, the collapse must result in a
star/disk structure in which the star has most of the mass and the
disk most of the angular momentum. Thanks to improved astronomical technology that has led to sharper
images, for instance with the Hubble Space Telescope (HST) and some of
the largest ground-based telescopes, some of these disks around nearby
young stars have been photographed. They are referred to as protoplanetary disks or
"proplyds". In the case of the Solar System, it is generally believed that the
disk around the young Sun some 4.5 billion years ago had a mass of a
few percent of the mass of the Sun and that its radius was less than
about 100 AU (15,000 million km; 1 AU is the distance between the
Earth and the Sun - 150 million km). Planets subsequently form in this disk, most probably
through
collisions, at first
between dust grains and as time goes by, between larger and larger
bodies. This picture provides a simple explanation why all planets
in our Solar System not only orbit the Sun in nearly the same plane
but also all move in the same direction. The nearly simultaneous
formation of the planets and other small bodies and the Sun is
actually supported by comparing the ages of the oldest Moon rocks and
the Sun. The basic challenge of planet formation therefore consists of
assembling in a disk orbiting a central star micron-sized or
smaller dust grains into planetary bodies measuring over 10000 km in
diameter, a growth by nearly a factor of 1013 in size,
or 1040 in mass! Since giant
planets are mainly
gaseous, their formation must take place before the gas supply in the disk is
exhausted. From studies of
disks around young stars, it is believed that the typical lifetime of
such disks is of the order of a few million years. Hence, paradoxical though it sounds, giant planets must be formed in less than ten million
years while the formation of terrestrial planets may take much
longer. On their way to becoming a planet, dust grains reach the size of
asteroids and comets which, if they can avoid
being incorporated in a larger object, are left behind like crumbs on
a table after a good meal.
Life in the Universe |
Last updated September 3, 2001
Planetary Formation